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Re: [Xen-devel] [PATCH v3 2/7] xen/nospec: Use always_inline to fix code gen for evaluate_nospec
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 29 Oct 2019 16:53:21 +0000
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- Cc: Juergen Gross <jgross@xxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Wei Liu <wl@xxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 29 Oct 2019 16:53:35 +0000
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On 25/10/2019 13:03, Jan Beulich wrote:
> On 23.10.2019 15:58, Andrew Cooper wrote:
>> evaluate_nospec() is incredibly fragile, and this is one giant bodge.
>>
>> To correctly protect jumps, the generated code needs to be of the form:
>>
>> cmp/test <cond>
>> jcc 1f
>> lfence
>> ...
>> 1: lfence
>> ...
>>
>> Critically, the lfence must be at the head of both basic blocks, later in the
>> instruction stream than the conditional jump in need of protection.
>>
>> When a static inline is involved, the optimiser decides to be clever and
>> rearranges the code as:
>>
>> pred:
>> lfence
>> <calculate cond>
>> ret
>>
>> call pred
>> cmp $0, %eax
>> jcc 1f
>> ...
>> 1: ...
>>
>> which breaks the speculative safety.
> Aiui "pred" is a non-inlined static inline here. There's no "optimiser decides
> to be clever" in this case imo - it all is a result of not inlining, when the
> construct in evaluate_nospec() is specifically assuming this wouldn't happen.
> Therefore I continue to think that the description is misleading.
>
>> Any use of evaluate_nospec() needs all static inline predicates which use it
>> to be declared always_inline to prevent the optimiser having the flexibility
>> to generate unsafe code.
> I agree with this part.
How about:
When the compiler chooses to out-of-line the condition calculation (e.g. by
not inlining a predicate), the code layout can end up as:
pred:
lfence
<calculate cond>
ret
call pred
cmp $0, %eax
jcc 1f
...
1: ...
which breaks the speculative safety, as the lfences are earlier in the
instruction stream than the jump in need of protection.
?
~Andrew
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